U.S. patent number 6,008,648 [Application Number 08/985,019] was granted by the patent office on 1999-12-28 for method for producing physical gradient waveforms in magnetic resonance imaging.
This patent grant is currently assigned to General Electric Company. Invention is credited to Paul E. Licato, Anton M. Linz, Graeme C. McKinnon, Jason A. Polzin.
United States Patent |
6,008,648 |
Linz , et al. |
December 28, 1999 |
Method for producing physical gradient waveforms in magnetic
resonance imaging
Abstract
An MRI system produces magnetic field gradients along physical
axes during a patient scan. The gradients are specified as logical
gradient waveforms in a pulse sequence and these are stored in a
logical vector table. A physical vector table is produced by
rotating amplitude values in the logical vector table, and the
resulting rotated amplitude values are used to calculate the
heating in each gradient axis of the MRI system.
Inventors: |
Linz; Anton M. (Mukwonago,
WI), Polzin; Jason A. (Lake Mills, WI), Licato; Paul
E. (Wauwatosa, WI), McKinnon; Graeme C. (Hartland,
WI) |
Assignee: |
General Electric Company
(Milwaukee, WI)
|
Family
ID: |
25531114 |
Appl.
No.: |
08/985,019 |
Filed: |
December 4, 1997 |
Current U.S.
Class: |
324/309; 324/307;
324/318; 600/425 |
Current CPC
Class: |
G01R
33/3852 (20130101); G01R 33/385 (20130101) |
Current International
Class: |
G01R
33/38 (20060101); G01R 33/385 (20060101); G01V
003/00 () |
Field of
Search: |
;324/309,313,318,308,307
;600/425 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Oda; Christine
Assistant Examiner: Shrivastav; Brij B.
Attorney, Agent or Firm: Quarles & Brady Cabou;
Christian G. Price; Phyllis Y.
Claims
We claim:
1. A method for producing gradient waveforms in an MRI system, the
steps comprising:
specifying a logical gradient waveform for each gradient to be used
in a pulse sequence;
detecting the break points in all the specified logical gradient
waveforms;
producing a logical vector table which stores the amplitude of each
specified logical gradient waveform at each detected break
point;
producing a physical vector table by rotating the amplitudes stored
in the logical vector table at each detected break point and
storing physical amplitudes for each break point; and
producing physical gradient waveforms for each gradient axis on the
MRI system by playing out the physical amplitudes stored in the
physical vector table at successive break points.
2. The method as recited in claim 1 which includes:
calculating from the physical amplitudes stored in the physical
vector table the heat produced by each gradient axis.
3. The method as recited in claim 2 which includes:
comparing the calculated heat produced by each gradient axis with a
preset limit; and
producing a message if the calculated heat exceeds the preset limit
for any gradient axis.
4. The method as recited in claim 1 in which the physical vector
table is produced by performing a matrix multiplication of the
amplitudes stored at each break point in the logical vector
table.
5. The method as recited in claim 1 in which the physical
amplitudes stored in the physical vector table are played out to
interpolators which produce substantially continuous gradient
waveforms.
6. The method as recited in claim 5 in which the physical
amplitudes are played out to the interpolators as the MRI system is
performing a pulse sequence in which magnetic field gradients are
generated using the gradient waveforms.
Description
BACKGROUND OF THE INVENTION
The field of the invention is nuclear magnetic resonance imaging
methods and systems. More particularly, the invention relates to
the production of gradient fields used in imaging and spectroscopy
pulse sequences.
When a substance such as human tissue is subjected to a uniform
magnetic field (polarizing field B.sub.0), the individual magnetic
moments of the spins in the tissue attempt to align with this
polarizing field, but precess about it in random order at their
characteristic Larmor frequency. If the substance, or tissue, is
subjected to a magnetic field (excitation field B,) which is in the
x-y plane and which is near the Larmor frequency, the net aligned
moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to
produce a net transverse magnetic moment M.sub.t. A signal is
emitted by the excited spins after the excitation signal B.sub.1 is
terminated, this signal may be received and processed to form an
image.
When utilizing these signals to produce images, magnetic field
gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically,
the region to be imaged is scanned by a sequence of measurement
cycles in which these gradients vary according to the particular
localization method being used. The resulting set of received NMR
signals are digitized and processed to reconstruct the image using
one of many well known reconstruction techniques.
The present invention will be described with reference to a variant
of the well known Fourier transform (FT) imaging technique, which
is frequently referred to as "spin-warp". The spin-warp technique
is discussed in an article entitled "Spin-Warp NMR Imaging and
Applications to Human Whole-Body Imaging" by W. A. Edelstein et
al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980).
It employs a variable amplitude phase encoding magnetic field
gradient pulse prior to the acquisition of NMR spin-echo signals to
phase encode spatial information in the direction of this gradient.
In a two-dimensional implementation (2DFT), for example, spatial
information is encoded in one direction by applying a phase
encoding gradient (G.sub.y) along that direction, and then a
spin-echo signal is acquired in the presence of a readout magnetic
field gradient (G.sub.x) in a direction orthogonal to the phase
encoding direction. The readout gradient present during the
spin-echo acquisition encodes spatial information in the orthogonal
direction. In a typical 2DFT pulse sequence, the magnitude of the
phase encoding gradient pulse G.sub.y is incremented
(.DELTA.G.sub.y) in the sequence of views that are acquired during
the scan to produce a set of NMR data from which an entire image
can be reconstructed.
The imaging gradients are produced by gradient amplifiers that
drive coils which produce magnetic fields having gradients directed
along physical axes. Typically, three gradient amplifiers with
corresponding coils produce gradient fields directed along three
orthogonal axes, x, y, and z. These are "physical" axes because
they are fixed with respect to the MRI system geometry.
MRI pulse sequences are defined by the gradient fields and the RF
fields that are to be produced during each NMR measurement. The
gradient fields are defined by gradient waveforms produced along
three orthogonal axes. For example, a slice-selecting gradient may
be applied along one axis; a phase encoding gradient may be
produced along another, orthogonal axis; and a readout gradient may
be produced along yet another orthogonal axis. These "logical" axes
may correspond to the physical axes on the MRI system for one
orientation of the slice or volume to be images (e.g. a transverse
plane), but as a general matter they do not. When the MRI pulse
sequence is performed on an MRI system, therefore, each logical
gradient field may be produced by the combination of one to three
of the physical gradient fields, depending on the prescribed
orientation of the image to be acquired.
When a pulse sequence is executed during a patient scan the logical
gradient waveforms are converted into physical gradient waveforms
for driving the gradient amplifiers on the MRI system. This is
performed by a matrix rotation of the logical gradient waveforms.
As described, for example, in U.S. Pat. No. 4,743,851, the logical
gradient is stored as a series of points which represent the
amplitude of the gradient at successive time increments (e.g. every
4 microseconds) during the pulse sequence. These points are read
out of memory in sequence and multiplied by three rotation factors
to rotate the logical gradient into each of the three physical
axes. For three logical gradient waveforms, this involves nine
multiplications and additions for every time increment of the pulse
sequence.
One of the practical limitations of MRI systems is the heat
dissipation capacity of gradient amplifiers and coils. Pulse
sequences can often be shortened to reduce total scan time by
increasing the amplitude of a gradient. Unfortunately, higher
amplitude gradients produced at a higher repetition rate also
produce more heat. As a result, gradient heating often limits the
extent to which a pulse sequence can be shortened.
Prior to the commencement of a scan it is desirable to calculate
the amount of gradient heating that will occur to insure that
limits are not exceeded. Since the logical gradient waveforms are
stored as a series of amplitude points, it is relatively easy to
integrate the logical waveforms over the duration of the pulse
sequence and determine the amount of heat each will produce. This
does not necessarily reveal, however, how much heating will occur
in the physical amplifiers and coils because the waveforms may be
changed considerably during the rotation into the physical axes.
The computations necessary to rotate the logical gradient waveforms
and calculate heating in each physical axis is substantial and not
usually performed in state-of-the-art MRI systems. As a result,
gradient pulse amplitudes are often unnecessarily derated to insure
that heating problems do not occur.
SUMMARY OF THE INVENTION
The present invention is an improved method for calculating the
physical gradients in an MRI system based on a prescribed set of
logical gradient waveforms. More particularly, the method includes:
determining break point times in the prescribed gradient waveforms;
forming a logical vector table in which the vector elements
comprise the amplitudes of the logical gradient waveforms, at each
successive break point time; and rotating the vector elements in
the logical vector table to form a physical vector table. The
physical vector table may be used to calculate heating in the
gradient amplifiers and coils so that the prescribed scan can be
optimized, and the physical vector table may be interpolated to
produce the physical gradient waveforms for driving the gradient
amplifiers on the MRI system.
A general object of the invention is to produce physical gradient
waveforms from a prescribed set of logical gradient waveforms. By
forming a physical vector table based on the amplitudes of the
logical gradients at each break point time, the physical vector
table can be produced with a few 3D vector rotations. The amount
and direction of the rotation is determined by the prescribed
orientation of the desired image with respect to the physical
gradient axes of the MRI system. The physical vector table can then
be used directly to estimate gradient heating and the same physical
vector table can be interpolated either off-line, or in real-time
to produce the physical gradient signals needed to drive the
gradient amplifiers.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an MRI system which employs the
present invention;
FIG. 2 is a graphic a representation of a set of logical gradient
waveforms prescribed for use in an imaging pulse sequence;
FIG. 3 is a flow chart of the procedures performed by the MRI
system of FIG. 1 to practice the preferred embodiment of the
invention;
FIG. 4 is a schematic representation of a logical vector table
produced in the procedure of FIG. 3; and
FIG. 5 is a block diagram illustrating the process of producing a
physical vector table which forms part of the procedure of FIG.
3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1, there is shown the major components of a
preferred MRI system which incorporates the present invention. The
operation of the system is controlled from an operator console 100
which includes a keyboard and control panel 102 and a display 104.
The console 100 communicates through a link 116 with a separate
computer system 107 that enables an operator to control the
production and display of images on the screen 104. The computer
system 107 includes a number of modules which communicate with each
other through a backplane. These include an image processor module
106, a CPU module 108 and a memory module 113, known in the art as
a frame buffer for storing image data arrays. The computer system
107 is linked to a disk storage 111 and a tape drive 112 for
storage of image data and programs, and it communicates with a
separate system control 122 through a high speed serial link
115.
The system control 122 includes a set of modules connected together
by a backplane. These include a CPU module 119 and a pulse
generator module 121 which connects to the operator console 100
through a serial link 125. It is through this link 125 that the
system control 122 receives commands from the operator which
indicate the scan sequence that is to be performed. The pulse
generator module 121 operates the system components to carry out
the desired scan sequence. It produces data which indicates the
timing, strength and shape of the RF pulses which are to be
produced, and the timing of and length of the data acquisition
window. The pulse generator module 121 connects to a set of
gradient amplifiers 127, to indicate the timing and shape of the
gradient pulses to be produced during the scan. The pulse generator
module 121 also receives patient data from a physiological
acquisition controller 129 that receives signals from a number of
different sensors connected to the patient, such as ECG signals
from electrodes or respiratory signals from a bellows. And finally,
the pulse generator module 121 connects to a scan room interface
circuit 133 which receives signals from various sensors associated
with the condition of the patient and the magnet system. It is also
through the scan room interface circuit 133 that a patient
positioning system 134 receives commands to move the patient to the
desired position for the scan.
The gradient waveforms produced by the pulse generator module 121
are applied to a gradient amplifier system 127 comprised of
G.sub.x, G.sub.y and G.sub.z amplifiers. Each gradient amplifier
excites a corresponding gradient coil in an assembly generally
designated 139 to produce the physical magnetic field gradients
used for position encoding acquired signals. The gradient coil
assembly 139 forms part of a magnet assembly 141 which includes a
polarizing magnet 140 and a whole-body RF coil 152. A transceiver
module 150 in the system control 122 produces pulses which are
amplified by an RF amplifier 151 and coupled to the RF coil 152 by
a transmit/receive switch 154. The resulting signals radiated by
the excited nuclei in the patient may be sensed by the same RF coil
152 and coupled through the transmit/receive switch 154 to a
preamplifier 153. The amplified NMR signals are demodulated,
filtered, and digitized in the receiver section of the transceiver
150. The transmit/receive switch 154 is controlled by a signal from
the pulse generator module 121 to electrically connect the RF
amplifier 151 to the coil 152 during the transmit mode and to
connect the preamplifier 153 during the receive mode. The
transmit/receive switch 154 also enables a separate RF coil (for
example, a head coil or surface coil) to be used in either the
transmit or receive mode.
The NMR signals picked up by the RF coil 152 are digitized by the
transceiver module 150 and transferred to a memory module 160 in
the system control 122. When the scan is completed and an entire
array of data has been acquired in the memory module 160, an array
processor 161 operates to Fourier transform the data into an array
of image data. This image data is conveyed through the serial link
115 to the computer system 107 where it is stored in the disk
memory 111. In response to commands received from the operator
console 100, this image data may be archived on the tape drive 112,
or it may be further processed by the image processor 106 and
conveyed to the operator console 100 and presented on the display
104.
For a more detailed description of the transceiver 150, reference
is made to U.S. Pat. Nos. 4,952,877 and 4,922,736 which are
incorporated herein by reference.
Referring particularly to FIG. 2, when a particular pulse sequence
is prescribed by the user of the MRI system, a set of logical
gradient waveforms are produced. In a 2DFT pulse sequence, for
example, a logical z gradient waveform 170 may be specified to
produce what is often referred to as a "slice selection gradient".
Similarly, a logical y gradient waveform 172 may be specified as a
"phase encoding" gradient, and a logical x gradient waveform 174
may be specified as a "readout" gradient. If these logical gradient
waveforms are applied directly to their corresponding x, y and z
gradient amplifiers 127, an image will be produced that has a set,
"reference" orientation with respect to the MRI system.
The present invention deals with the very common situation in which
the user has prescribed an image that is oriented at an angle with
respect to this reference orientation. As is well known in the art,
this requires that the three logical gradient waveforms 170, 172
and 174 be rotated by a corresponding angle. For three gradients,
this rotation is performed by a 3 by 3 matrix multiplication as
described, for example, in U.S. Pat. No. 4,743,851. Each logical
gradient waveform is played out and separately rotated by a matrix
multiplier to produce components for each physical gradient axis.
As a result of such rotation, the logical z slice selection
gradient waveform 170 may be produced by any one or combination of
two or three of the physical gradient amplifiers on the MRI system.
The same is true of the other logical gradients.
Rather than rotating each logical gradient waveform to the
prescribed image orientation, the teaching of the present invention
is to combine the three logical gradient waveforms into a
three-dimensional vector representation of the logical gradients.
Referring particularly to FIG. 3, once a particular set of logical
gradient waveforms have been prescribed, as indicated at process
block 200, the break points in each waveform are detected as
indicated by process block 202. Referring again to FIG. 2, the
break points are those times indicated by the vertical dotted lines
204 at which one or more of the logical gradient waveforms 170,
172, or 174 have a break, or discontinuity, in their form. Each
waveform is viewed as a series of continuous segments and the break
points are where two segments join. The gradient waveforms are
prescribed by the amplitude at each break point. In the preferred
embodiment only linear segments are shown, however, other shapes
such as cubic spline or circular are also possible.
After the break points are detected, a logical vector table 206 is
constructed as indicated at process block 208. As shown in FIG. 4,
this logical vector table 206 is formed by storing the time of each
detected break point and the amplitude of each logical gradient
waveform at that time. When the time corresponds to a break point
in a logical gradient waveform this task is trivial since the
amplitude is specified at each break point. However, when the time
occurs in the middle portion of a gradient segment (e.g. the point
210 in FIG. 2) the amplitude of the gradient must be interpolated.
In the preferred embodiment in which only linear segments are used,
linear interpolation between the amplitudes specified at the ends
of the intersected segment is performed. For each detected break
point, therefore, the time at which the break point occurs and the
amplitude of each logical gradient waveform x, y and z is stored in
the table 206. Each entry in the table can be viewed as a
three-dimensional vector in which its x, y and z components
reference a logical coordinate system.
Referring again to FIG. 3, the next step in the process is to
rotate the three-dimensional vectors in the logical vector table
206, as indicated by process block 212.
As shown in FIG. 5, this rotation is accomplished with a 3 by 3
matrix multiplication 214 of each vector in the table 206 to
produce a corresponding rotated vector that is stored in a physical
vector table 216. If the three-dimensional vector in table 206 at
time t.sub.i is represented by x.sub.i, y.sub.i, z.sub.i, the
corresponding physical vector x.sub.i ', y.sub.i ', z.sub.i ', in
the table 216 is thus produced as follows: ##EQU1## The physical
vector table 216 is structured, the same as the logical vector
table 206, but the values of the x, y and z components are
different to reflect the fact that they reference the physical
gradients on the MRI system.
As indicated at process block 218 in FIG. 3, the heating produced
by the gradient waveforms may now be accurately calculated. The
entries in the physical vector table are the amplitudes at each
break point of the gradient waveforms applied to each gradient
amplifier on the MRI system. The area under each of these waveforms
can be easily calculated using the break point amplitudes and the
heating load can be estimated from this figure. If the heating load
on one or more of the axes exceeds a preset limit, the system
branches at decision block 220, an appropriate message is displayed
to the operator at 222, and the system loops back to process block
200 where the gradient waveforms can be re-specified.
When the final gradient waveforms have been specified the contents
of the physical vector table 216 may be played out as indicated at
process block 224 to control the gradient amplifiers 127 during the
scan. This can be accomplished in a number ways. First, the break
point amplitudes of each physical axis gradient waveform may be
interpolated off-line to produce a set of incremental amplitude
values which are stored. These stored amplitude values indicate the
gradient amplitude at each time increment (e.g. every 4
microseconds) during the pulse sequence and they are read out of
memory in sequence and applied to the gradient amplifiers 127 as
the pulse sequence is performed by the pulse generator module 121.
A second method is to play out the break point amplitudes from the
physical vector table 216 while the pulse sequence is being
performed. In this embodiment the interpolation is performed in
real-time and is best accomplished by the addition of a 3-axis
interpolator to the input of the gradient amplifiers 127. The pulse
generator module 121 thus reads the break point amplitude vectors
out of the table 216 and the gradient amplifiers 127 perform the
interpolation necessary to provide 3 axis, continuous control over
the three gradient fields.
It should be apparent to those skilled in the art that the break
point amplitude values stored in the physical vector table 216 can
also be used for other purposes. Eddy current compensation methods
require the shape of the physical gradient waveform and its
derivative. These are easily produced from the break point
description in the table 216 and may be used to calculate constants
for Eddy current compensation circuits used in the gradient
amplifiers 127. Similarly, the break point description of the
gradient waveforms may be used by adaptive ECG filtering circuits
to calculate the necessary constants.
* * * * *